1. Field of the Invention
The present invention relates to image forming apparatuses, and more particularly to an image forming apparatus including a recording head for jetting liquid droplets.
2. Description of the Related Art
There are image forming apparatuses such as printers, fax machines, copiers, plotters, and multifunction peripherals including these functions. An inkjet recording device is known as an example of a liquid jet recording type image forming apparatus including a recording head for jetting ink droplets. A liquid jet recording type image forming apparatus jets ink droplets from a recording head onto a sheet that is being conveyed, for forming images (recording, printing, etc., are also used as synonyms of forming). There are serial type image forming apparatuses for forming images by jetting liquid droplets while moving the recording head in the main scanning direction. There are also line type image forming apparatuses using line type heads for forming images by jetting liquid droplets without moving the recording head.
In the present application, an image forming apparatus is an apparatus for forming images by jetting ink onto media such as paper, threads, fiber, cloth, leather, metal plastic, glass, timber, and ceramics (also including a simple liquid jetting device). Forming images does not only mean to form images having meaning such as characters and figures onto media, but also means forming images without any meaning such as patterns onto media (including liquid jetting devices that simply jet liquid droplets onto media). In the present application, ink is not limited to ink per se, but ink is a collective term of all kinds of liquid with which images can be formed, including recording liquid, fixing-processing liquid, liquid DNA samples, and patterning materials. In the present application, a sheet may be a sheet other than a paper sheet, such as an OHP sheet or cloth; a sheet is a collective term of all kinds of sheets to which ink droplets can adhere, including a recording medium, a recording sheet, etc. An image is not limited to a planar image; an image may be three-dimensionally formed on the sheet, or a three-dimensional object may be formed on the sheet.
Examples of known liquid jetting heads used as recording heads include the following. One example is a piezoelectric type head that jets liquid droplets by using a piezoelectric actuator to change the position of an oscillating plate so that the volume in the liquid chamber is changed and the pressure is increased. Another example is a thermal type head that jets liquid droplets by increasing the pressure in the liquid chamber with bubbles that are caused by heat generated by a heating element, which is heated by electric currents applied inside the liquid chamber.
For such a liquid jet recording type image forming apparatus, there is demand for increasing image formation throughput, i.e., for increasing the speed of image formation. Accordingly, in the image forming apparatus, ink is supplied from a high-volume ink cartridge (main tank) that is set in the main unit of the image forming apparatus, to a sub tank (or buffer tank) above the recording head, through a tube. By using this method of supplying ink through a tube (tube supplying method), the carriage part can be made light-weight and compact, so that the size of the apparatus can be significantly reduced, both in terms of the structure and the driving system.
In the tube supplying method, the ink that is consumed for forming images with the recording head is supplied to the recording head from the ink cartridge through a tube. If this tube is a thin tube that is highly flexible, the fluid resistance is high when the ink flows through the tube. Consequently, the ink may not be properly supplied in time for jetting ink, and jetting failures may occur (also referred to as refill failures). Particularly, in a large size machine used for printing images onto wide recording media, the tube is inevitably long, and therefore the fluid resistance is high. Furthermore, when high-speed printing is performed, or when highly viscous ink is used, the fluid resistance is high, and the ink may not be properly supplied to the recording head.
Conventionally, there is known a method as disclosed in patent document 1. Specifically, pressure application on the ink in the ink cartridge is maintained, and a differential pressure regulating valve is provided on the upstream side of the path for supplying ink to the recording head. Accordingly, the ink is supplied when the negative pressure in the sub tank becomes higher than a predetermined value.
Furthermore, there is known a method as disclosed in patent document 2. Specifically, a negative pressure chamber that achieves negative pressure with the use of a spring is provided on the upstream side of the recording head. The ink supplying pressure is actively controlled by sending ink to the negative pressure chamber with the use of a pump. In a method as disclosed in patent document 3, the ink supplying pressure is also controlled with the use of a pump, although in this case there is no negative pressure chamber.
Meanwhile, there is also a method of achieving negative pressure with a simple structure. Specifically, an ink cartridge connected to the atmosphere and a recording head are connected by a tube. Simply by positioning the ink cartridge below the recording head, negative pressure is achieved by the water head difference.
This water head difference method is far more simple than the method of constantly applying pressure with the use of a valve coupled with negative pressure or the method of providing a negative pressure chamber and sending liquid to the negative pressure chamber with the use of a pump. Nevertheless, negative pressure can be stably achieved with the water head difference method. However, the problem with the water head difference method is that pressure loss may be caused by the resistance in the tube as described above.
Patent document 4 discloses a technology for solving the problem of pressure loss of an ink supplying system in which negative pressure is achieved according to the water head difference. Specifically, a pump is provided in the tube connecting the recording head and the ink cartridge. A bypass path is provided for connecting the upstream side and the downstream side of the pump. A valve is provided in the bypass path. The degree of opening the valve in the bypass path is appropriately controlled in accordance with the printing operation to maintain an appropriate level of pressure.
Patent Document 1: Japanese Patent No. 3606282
Patent Document 2: Japanese Laid-Open Patent Application No. 2005-342960
Patent Document 3: Japanese National Publication (Japanese translation of PCT application) No. H5-504308
Patent Document 4: Japanese Laid-Open Patent Application No. 2004-351845
However, although the technology disclosed in patent document 1 solves the problem of refill failures as described above, the mechanism for controlling the negative pressure is complex. Furthermore, the valve coupled with negative pressure needs to have good sealing properties. Furthermore, pressure is constantly applied in this method, and therefore all of the connection parts in the ink supplying path need to be highly airtight. Thus, in the event of a failure in the system, the ink may blow out.
In the technologies disclosed in patent documents 2 and 3, the pressure is actively controlled with the use of a pump, and therefore the amount of liquid conveyed by the pump needs to be accurately controlled in accordance with the consumption amount of ink. Therefore, it is necessary to perform feedback control with the pressure in the negative pressure chamber. Furthermore, when these technologies are applied to an image forming apparatus in which plural ink types of different colors are used, the pump needs to be controlled according to the different colors. Therefore, complex control operations and a large-sized apparatus are required.
In the technology disclosed in patent document 4, when this technology is applied to an image forming apparatus in which plural ink types of different colors are used, the pump needs to be controlled according to the different colors. Therefore, a large-sized apparatus is required.